Teenage parents are the sitting duck of electioneering: children offer an ideologically neutral, uncomplicated area for "improvement", and the data is there to show how important the early years are – especially now, with the Millennium Cohort Study's results showing ­babies who are behind at nine months are still left behind aged five. The ­difficulty comes in trying to establish what type of parents are the most ­inadequate. You can't do it by class: that would be either bigoted or ­revolutionary. It is easier to do it by age; and easier still to blame teenagers. The trouble is, many of the preconceptions about teenage ­parents, on which actual policy is based, are wrong. Teenage Parenthood: What's the Problem? is a collection of essays, ­published this week, debunking so many of the ­negative stereotypes that it's almost embarrassing to read.

The active avoidance of teenage pregnancies was enshrined into policy in 1999 by Tony Blair. Simon Duncan, professor of comparative social policy at Bradford University, says that, in fairness to the early Blair years, much of the research contradicting his policy hadn't then been conducted. No such excuse holds for the 2007 report by the Department for Children, Schools and Families, which reiterates the same ideas. A spokesperson for DCSF gave out the same line again, when asked by the Guardian recently to comment on the book: "While many teenage parents manage very well, they and their children are more likely to suffer health, emotional and economic problems. As the vast majority of teenage pregnancies are unplanned, our strategy focuses on giving young people the knowledge, skills and confidence to make positive and informed choices about sexual activity and parenthood."

This should ring alarm bells even for people with just a cursory interest in the subject. One in three births in the UK is unplanned, and teens account for just over 10% of mothers. So even if 100% of teenage pregnancies are unplanned, that still makes up nothing like the total. Yet you don't see initiatives telling the older accidental mother she needs more "knowledge, skills and confidence", that she hasn't made a "positive" choice. Besides being inappropriate and none of the state's business, it would be rude.

It was a New Labour initiative also, Jan McVarish (another of the report's authors) points out, that turned under- 18-year-olds into a problem; previously, it had just been under-16s – who were already a problem, being below the age of consent.

A range of the ways in which this policy is contradicted by the research is compellingly laid out in the book. But one of the laziest arguments is right there in the DCSF statement – that the children of teenagers were "more likely to suffer health, emotional and economic problems". If they're more likely than other children in similar financial situations, then that's a good case, but if they're just "more likely" than children born to richer parents – and this is what the figures show – then the problem isn't youth. And if we accept poverty rather than age as the driver then a government of any persuasion has to stop telling selected individuals to stay childless and start to address financial inequality.

At a conference to launch the book, a government adviser, Roger Ingham, defended the policy. He claimed it had improved a lot of teenage parents' lives, with funding initiatives and research – not a bad place to start "while we're waiting for social deprivation to be addressed". But that's the point. Policies like this are not interim measures to keep us busy while we wait for wealth redistribution. This is what happens instead – small groups are identified as the generators of social problems, then ring-fenced for special treatment. So, on a number of levels, progressive opinion should be against this, not least because it is sleight of hand, and draws attention away from the task that would mean something. More importantly, it's flaky: if you're going to victimise a group, you have to at least have data on your side.

If the Labour party looks like the big culprit here it's because it's in office. Cameron is appalling on this – almost nothing he says about teenage parents adds up. In this week's document, Labour's Two Nations, the Tories made the well-publicised claim that in the 10 most deprived areas "54% of girls are likely to fall pregnant before the age of 18"; the actual figure is 5.4%. When Baby P was killed, Cameron tried to make a case that he had died through Labour's failure to bring down teenage pregnancy rates. Wrong twice: Baby P's mother was 27; Labour has brought down teenage births by nearly 24% – and this birthrate hasn't been so low since the 1950s.

There is something rotten in a political class that will believe anything of this country's 10 most deprived areas. And something unseemly about policymaking that makes such florid rhetorical use out of a group that, in the main, can't vote. But in the end it's not the snobbery that's so depressing, it's not the tacit understanding between political players that poor people are less hassle when they're childless. It's the sloppiness. Sleight of hand is bad enough, but these magicians aren't even concentrating.

• This article was amended on 18 February 2010. The original version misspelled Roger Ingham's name as Roger Ingram. This has been corrected